When the Operation Easter detectives opened the door of the second bedroom in an ordinary house in Cleethorpes last month, they suspected they would find a shoebox or two of wild birds' eggs. Officers did not imagine they would discover 20 polystyrene fish crates, several biscuit tins and a suitcase, containing a collection of 7,707 eggs. Nestling between layers of cotton wool were the dainty speckled pale green and blue of blackcaps, corn buntings and yellowhammers, just like Cadbury's mini-eggs; the larger, blotched chestnut red of the osprey; goshawk eggs the colour of the moon. Each aquisition was meticulously recorded in notebooks; each egg painstakingly labelled in spidery black ink and its insides blown out through a tiny hole drilled in its side. For the collector, these dead things had clearly been a life's work.

Despite being fined, jailed and derided as deviants by birdwatchers, a small band of obsessive collectors known as eggers continue to illegally take the freshly laid clutches of wild birds. Last year, 54 egg collecting incidents were reported to the RSPB. The 12 reports of illegal sales of wild birds' eggs was the highest figure since 1995. The seizure in Cleethorpes came shortly after the unrelated case of Michael Barclay, 68, a well-to-do landowner, who pleaded guilty to buying prohibited wild birds' eggs and taxidermy and was sent to prison for four months.

Wildlife experts fear the recent arrests reveal only a fraction of the true picture. "The guy [from Cleethorpes] has never been on our radar before and he's got nearly 8,000 eggs," says Mark Thomas, one of the RSPB's five investigations officers. "How come nobody has ever caught this bloke? How come nobody has ever rung up with suspicions about this guy? That's what worries us. How many more are out there?"

Virtually incomprehensible in most other countries, egg collecting is a peculiarly British impulse, a residue of a colonial era when Victorian explorers brought back booty from around the globe and oology, the study of eggs, was a respectable branch of ornithology. Known for his fiery temper, the eminent Victorian collector the Rev FCR Jourdain would be publicly challenged to identify eggs by his rival, PF Bunyard. Oologists earnestly debated bird species at annual dinners held by their charity, which they named the Jourdain Society. And small boys would embark on a lifelong passion for birds by snaffling eggs from hedgerows.

After the second world war, collecting became more furtive. Guardian country diarist Harry Griffin referred to collectors as "the cloak-and-dagger men of the fells" in a 1952 column. Taking wild eggs was made illegal in 1954, but collecting continued. The extinction in the 1980s of the red-backed shrike, which had the misfortune to lay pretty, speckled eggs, was attributed to collectors. "It was a little bit cops-and-robbers in the 70s and 80s, but it peaked in the late 1990s," says Thomas. Increasingly professional eggers cracked hi-tech security operations that were attempting to protect imperilled populations of golden eagles, white-tailed sea eagles and ospreys.

By the late 90s, the RSPB was receiving, on average, one report of nest theft every day. Tayside police set up Operation Easter, a rolling nationwide investigation that is still active today. In 2001, it targeted a hard core of 130 eggers. There was "the Hoover", so called because he would pitch up at a tern colony and take hundreds of eggs, and "the Abbott and Costello of egg collecting", Jamie and Lee McLaren, caught in the Orkneys in 1997 with videos of each other robbing nests (in one, a fulmar vomited over their shoes in an attempt to protect its offspring). On another occasion, a warden found his name and "fuck off" written in lipstick on hen eggs plonked in the raided nest of a peregrine falcon.

Collectors were brazen because they knew they were untouchable. Allowing only for fines related to the convicted's ability to pay, the law was no deterrent. Eggers such as Colin Watson, who died last year falling from a 40ft larch he was climbing to examine a nest, clocked up repeated convictions during the 80s and 90s. Despite paying fines totalling nearly £6,000, Watson persisted, on one occasion trying to fell a tree in Scotland with a chainsaw to reach an osprey's nest.

Rather than their prey, it was eggers who became threatened with extinction when the law changed in 2001. Collectors could now be jailed. One of the most notorious eggers, who inspired a small following in Coventry, handed over his vast collection to police so he would not be prosecuted. "That was the turning point," says Thomas. "He was almost an idol of the egg collectors. When they saw him give away his collection they thought this new law is really serious."

Derek Lee, 39, was the first egger to be jailed for his obsession. Two fines failed to curtail the secret hobby he began when he was eight, captivated by a tawny owl chick his brother's friend snatched from a nest. "We went to the local park and took common eggs such as blackbirds' and song thrushes'," he says. Most gave up when they got older, but Lee graduated to rarer species. "For me, it gradually got to be an obsession. When I left school at 16, I had a bit of spare time and a little money so I travelled elsewhere to pick up a kestrel or sparrowhawk egg. Then the next challenge was a buzzard. Eventually, I came across peregrines and red kites."

Like many eggers, Lee confided only in a couple of close friends who shared his passion. They would meet in the winter and methodically plan raids in the spring when eggs were freshly laid (nearly hatched eggs cannot easily be blown and preserved). "It's a bit like a bank robbery. You've got to do your own work at getting certain eggs. You can't just go into a field and pick them up," he says.

Travelling from his home in Greater Manchester to Wales or the Lake District, Lee would share information about where birds were nesting - and wardens were waiting. Savouring the adrenaline rush of outwitting the authorities, Lee would pose as an ordinary birdwatcher and chat to wardens, "getting information from them without them knowing I was taking eggs".

When they got their hauls home, Lee and his confidants would note the Latin name of each bird and where they found the egg. Then they would squirrel it away. "You'd look on it again occasionally and reminisce about what you'd been up to, but most of the time it would be tucked away. You couldn't show it off to anybody because you wouldn't know who you were talking to - their dad might be a police officer. It's not something you'd broadcast."

He still remembers the last egg he ever collected - a woodcock - and the visit at his home at 8am, a few months after the law changed. The police were at his front door and back, brandishing search warrants. They discovered hundreds of eggs. Lee was given a six-month prison sentence. "It was a bit of a shock. When I came out of prison I decided to give up," he says.

Others, always men and often working class, followed Lee to jail. Carlton D'Cruze, who kept journals describing how a sea eagle on the Isle of Mull broke one of her own eggs in a desperate attempt to defend her nest from him, received six months in 2002. Anthony Higham, found with video footage showing him stealing red-throated divers' eggs on the Orkneys, was jailed in 2003. Daniel Lingham, a carpenter living in a caravan filled with hundreds of nightingale and nightjar eggs, was put away last year. Custodial sentences are a particularly powerful deterrent, according to Thomas, because fellow prisoners see egg thieves as particularly odd deviants. "We know that people stealing birds eggs don't get a very good time inside. It's almost seen as paedophilia; it's a crime committed by weirdos in the eyes of the 'normal' criminal set," he says.

Why do eggers still do it? While some, such as Barclay, pay money for collections, the desire to raid nests is rarely financial. "It's an individual trophy that relates to that person, the location they took it from and the work they did behind the scenes to find the nest," says Thomas. "It's about a guy in Coventry leaving at night and driving to Scotland, walking 15 miles through fields of snow, abseiling down the crag, knocking the golden eagle off its nest, stealing the eggs, blowing them, burying them in tins in the ground, driving back to Coventry with nothing and waiting until autumn to return when birds are not breeding so they won't get caught in transit."

Some eggers' mania for collecting extends to drawers of seashells or boxes of toy soldiers. Most of those jailed have speeded their own journey to the cells thanks to the evidence of their own index cards or coded computer files carefully detailing every clutch and theft. An obsessive-compulsive attention to detail can obscure reality. On one police raid, officers were amazed to find an egger reminding them to wipe their feet and not spoil his carpet.

Others seem addicted, gripped by an obsession they can no longer master. When the knock finally comes on some collectors' doors "there's this dawning realisation of what they've been doing for 20 years or more," says Thomas. "We've had some break down and cry and say, 'I'm so glad you've finally come, this has been controlling me for years. The breeding season comes and I can't stop going out and collecting eggs.'"

Lee testifies to its addictive quality. "There are quite a few who are obsessed with it. Every single spring and summer they can't wait to get out. If you put a child in a chocolate factory their eyes light up with excitement. It's like that. When spring and summer come, the eggers are on edge. They're like big kids."

While Lee now sticks to bird watching - "It's not quite the same but you enjoy the birds flying about" - he claims to know several eggers who are still active. "All the collectors are thinking about packing in, but you'll always get a few hard-core collectors who will carry on. You're not going to stamp it out completely. I know of two who are still taking them. One got sent down for it and he came out and he was still taking them. Some people don't learn their lessons."

Eggers are getting smarter at hiding their collections. "They'll put them somewhere where the RSPB can't find them - in a lock-up garage or in their place of work," says Lee. When police raided Barclay's grand home, Hanworth Hall, in Norfolk, they discovered a secret door in a wardrobe leading to a concealed room containing empty cabinets ready for egg collections. Most seizures now come from tip-offs, often from disgruntled wives who have played second fiddle to their husband's obsession for decades or from others in the egging community who betray their friends and rivals.

According to Alan Stewart, wildlife and environment officer for Tayside police, Operation Easter now tracks 70 suspected collectors, down from its peak of 130 five years ago. While some have given up, others, he says, have been driven abroad, taking cheap flights to Madrid, for instance, where they can target Spanish imperial eagles and post the eggs home to a friend's address. The National Wildlife Crime Unit is now liaising with Interpol to tackle egg collectors who go to Europe. "We can't relax our guard," says Stewart.

Does it mattter? Some might argue that the oologists, who still gather at dinners organised by the Jourdain Society - now without its charitable status - are harmless eccentrics, even if some, reportedly, have convictions for egg collecting. Apologists claim that responsible collectors only take freshly laid clutches and most birds simply lay a second clutch. (Not so, says the RSPB, which points out that big birds of prey only ever lay once a year, while unscrupulous thieves commonly take as many clutches of eggs as a bird will lay in a breeding season.)

But, ironically, egg collections have helped preserve some bird species by assisting vital scientific discoveries. In a pioneering study, the naturalist Derek Ratcliffe pinpointed the cause of a near-catastrophic decline in birds of prey by comparing peregrine eggs from historical collections with eggs he tested in the 1960s. Finding the older eggs were heavier, he proved that pesticides were causing the birds to produce thinner eggshells that often cracked before they could hatch. Some conservationists argue that a form of egg collecting must continue for the good of future environmental science.

An inescapable melancholy hangs over the spoils of the Cleethorpes raid, now being counted in an RSPB warehouse in Bedfordshire. So much energy and care has gone into gathering and recording these eggs, each one a bird that has never flown. "It would be good to think that this collection is the last major one seized in the UK," says Thomas. "It's very encouraging to know that there's not an increase in young people taking birds' eggs. The majority are in their 40s. Maybe in a generation's time, the day of the egg collector will be over".